This invention relates to a method for stranding profile strands. The products obtained by the method of this invention typically include composite aerial wires such as aerial ground wires and aerial transmission wires, but they also include stranded wires such as ropes. In addition, this invention is advantageous particularly when applied to an optical composite aerial ground wire having an optical transmission function.
For example, an aerial ground wire which has been used conventionally is formed of an aluminum covered steel wire and/or aluminum alloy wire of circular cross section, and comprises a single central wire and several, e.g., 6 strands twisted around the central wire. Recently, however, with the spread of optical transmissions, an attempt has been made to make efficient use of an aerial ground wire by providing such a wire with an optical transmission function.
Such aerial ground wire having an optical transmission function uses an optical fiber conductor as the central wire material. There are various types of fiber optical conductors, but most of these conventional optical conductors comprise a pipe of aluminum or its alloy which receives an optical fiber core of one or more optical fibers. Although each optical fiber forms an optical conductor, several such fibers forming a core may still be referred to as an optical conductor. However, the tensile strength of an optical fiber conductor is not so high as that of an ordinary wire. Thus, the use of an optical fiber in a conductor results in a decrease in the strength of the aerial ground wire as a whole. To obtain the strength necessary for an aerial ground wire, one could increase the diameter of strands to be used. If, however, such procedure is simply put to practice, the apparent diameter of the aerial ground wire in its entirety increases and hence the wire is liable to receive an increased wind pressure; thus, from the standpoint of use of an aerial ground wire, the result is not so desirable.
As a method of increasing the cross-sectional area and the strength of each strand while decreasing the outer diameter of the entire aerial ground wire, the following suggestion has been made: the cross-section of strands to be stranded or twisted together to form an optical fiber conductor, should be made substantially a sector so that the outer surface of the aerial ground wire obtained is substantially a cylindrical surface. The sector referred to herein is a shape which is obtained when an area defined between two concentric circles is cut by radially extending lines. If a stranded wire is formed of strands each having such cross-sectional shape, not only is the tensile strength increased without increasing the outer diameter of the stranded wire so much, but also there is obtained a concomitant effect that the ambient pressure on the optical fiber conductor passing through the center is relieved by the bridge effect of the strands forming the stranded wire layer.
However, profile strands having the mentioned sector cross-section encounter some problems during stranding. The conventional stranding methods are roughly classified into two types. One first type stranding is the so-called "untwisted" type. The stranding machines for this first type of stranding include a "planetary type stranding machine", a "tubular type stranding machine" and a "take-up rotary type stranding machine having an untwisting function". The second stranding type is the so-called "non-untwisting" type. The stranding machines for this type of stranding include the "rigid type stranding machine" and the "take-up rotary type stranding machine having no untwisting function".
In the "untwisted" type, e.g., in a planetary type stranding machine, bobbins for supplying strands are caused to perform a planetary motion around the central wire material while maintaining their axes directed in a fixed direction. Therefore, the strands will be spirally wound around the central wire material while their inclined attitudes on the cross-sections of the strands to be stranded together around the central wire material, are maintained constant. If the cross-section of the strands is circular, even the use of this type of stranding makes it possible to arrange the strands regularly around the outer peripheral surface of the central wire material, but in the case of profile strands having a profile cross-section such as a sector, this "untwisted" type cannot be simply used.
In contrast thereto, with the "non-untwisted" type of stranding, said profile strands can be stranded together in a state in which they are in close surface contact with the outer peripheral surface of the central wire material. However, the strands stranded together by this type will have large stresses remaining therein, and unless these stresses are removed, it is impossible to obtain a stranded wire having a satisfactory performance. That is, what is required of a stranded wire is freedom from the "loosening" of the strands, freedom from the rotation (untwisting) of the entire stranded wire, and freedom from the "undulation" of the stranded wire, meaning that the stranded wire should be straight as a whole. However, where stranding is performed by the "non-untwisted" type, the aforesaid requirements for stranded wires cannot be met due to residual stresses in the strands. For this reason, where stranding is performed by the "non-untwisted" type, it has been a usual practice to pass the stranded wire through a post-forming step subsequent to the stranding or twisting step so as to remove the residual stresses in the strands. This post-forming step is performed by post-forming rollers externally pressing the stranded wire.
Even if this post-forming step is performed, this does not necessarily mean that all the problems caused by the "non-untwisted" type can be solved. First, in the case of a stranded wire of high tensile strength, such as is formed of steel strands, the residual stresses cannot be completely removed by a mere post-forming step. Particularly in the case of a stranded wire forming the aforesaid optical fiber conductor and using a central wire material, the optical fibers can be damaged by this post-forming step. That is, the pressure applied by the post-forming step can deform the pipe which forms the outer periphery of the optical fiber conductor, thus crushing the optical fibers in the interior, or the rollers used in the post-forming step produce a tensile force which can break the optical fibers in the interior.